Although cutaneous basal cell carcinoma (BCC) and squamous cell carcinoma (SCC) are the most common cancers in the US, specific mechanisms driving human epidermal transformation are not well understood. Ras is the most commonly mutated proto-oncogene in human cancer and has been implicated in human SCC. A model for human SCC has recently been developed in our laboratory employing Ras and cyclin dependent kinase 4 (CDK4) to create a malignant epidermal cell population that accurately mimics naturally occurring human SCC. This transformation is driven by only two genetically defined elements, and represents an ideal system with which to accomplish the primary goal of advancing a more complete understanding of specific mechanisms mediating Ras driven human epidermal neoplasia. First, the relative contribution of each of the three major Ras downstream effector arms in driving epidermal neoplasia with CDK4 will be identified by activating each signaling pathway individually. Constitutively active Ras mutants known to activate either the Raf, PI3K, or RalGDS pathways will be expressed with CDK4 in human keratinocytes and grafted onto immunodeficient mice where effects on epidermal growth and differentiation will be examined. Individual pathways will also be activated in limited combinations as multiple effector arms may act in concert to reproduce the completely transformed invasive SCC. Second, the necessity of individual Ras effector arms will be defined further through both genetic and pharmacologic blockade of specific components in the Ras signaling pathways. This will identify key points for potential therapeutic intervention. Finally, new technologies utilizing small tissue penetrating polypeptides will be developed to interact specifically and inhibit critical components of the Ras signaling pathway in human epidermis. At the end of the funding period, we hope to have defined specific mechanisms through which Ras drives epidermal neoplasia to advance an understanding of human tissue tumorigenesis and to guide development of future targeted molecular therapies for cancers of the skin and other tissues.